The present invention relates to a frequency tracking system and, more particularly, to a system for phase locking onto a horizontal scanning frequency pulse used in conjunction with personal computers (PC) monitors and the like.
Television receivers and PC monitors are driven by a horizontal and vertical scanning frequency pulse commonly referred to as horizontal and vertical sync pulses. More specifically, a prior art PC monitor system is available that can lock to horizontal scanning sync pulses the frequencies of which can vary from 15.5 Khz to 35 Khz. The conventional technique to achieve lock in to the horizontal sync pulse uses a phase locked hoop (PLL) much like the common television receiver but with the addition of a frequency to voltage converter. The frequency to voltage converter sets the center range of horizontal voltage controlled oscillator (VCO) of the phase lock loop depending on the frequency of the input synchronization pulses.
In the prior art PC monitor system the frequency to voltage converter is operated in an open loop mode and only provides a frequency offset to the VCO. This means that the frequency to voltage converter and the VCO of the PLL must be very accurate to insure the PLL will lock over the whole frequency range of the applied horizontal sync pulses. The pull-in range of the PLL must be relatively narrow to prevent false lock conditions that can be caused by harmonics and subharmonics of the synchronizing signal.
To ensure linear tracking and accuracy between the frequency to voltage converter and the VCO, the prior art system utilizes many discrete components and requires adjustments of some of these components.
Hence, a need exists for a simplified frequency tracking system that is suitable for use in PC monitors and the like which can be integrated in a monolithic integrated circuit which does not require precision components thereby eliminating the need for large numbers of external and discrete components.